Malaria Transmission

#### **Chapter 3**

### One Health Perspective of Malaria Transmission

*Jonas Bedford Danquah and Jennifer Afua Afrifa Yamoah*

#### **Abstract**

Global efforts towards malaria control and elimination are promising. Despite this, current alterations in transmission continue to modify and frustrate such effort. In 2020 and 2021, malaria transmissions increased significantly. While 2021 showed a decline in malaria deaths by 6000 (1%), the numbers were still 51,000 (9%) higher than malaria deaths in 2019. Two-thirds of the contributing factors were attributed to the COVID-19 pandemic, thus demonstrating the capability of future pandemics and zoonotic diseases to stagger or derail earned achievements towards malaria elimination. Compounded by zoonotic and environmental factors that promote malaria transmission, there will be a need for relevant modelling and an update on current and past disease distribution information and will also be required to shape policy actions and to improve public health decision-making on malaria. These will help strengthen the evidence for the adoption of relevant implementation strategies to aid the 2030 vision of eliminating malaria a reality.

**Keywords:** zoonotic malaria, environmental transmission, non-human malaria, endemicity, pandemic

#### **1. Introduction**

Malaria is an ancient disease of public health concern, which has scourged and threatened humanity for many centuries till today. The nemesis of it was estimated to be over 400 million deaths recorded globally within the 20th century alone [1]. Archaeological artefacts and relics of clay tablets, papyrus, religious and medical texts, parasite DNA detections in mummified remains, modelled ancient seasonal and climatic indicators are chest of trove suggesting that the disease has since been known perhaps during the Palaeolithic and Neolithic era [2–6]. The means of disease transmission remained elusive until the discovery of the malaria parasite by Charles Alphonse Laveran in 1880 and the related transmission vector by Sir Ronald Ross in 1897.

Preceding Laveran and Ross's findings are remarkable leads. These include a 270 BC Chinese canonic account in the "*Huangdi Neijing*" describing periodic fevers associated with enlarged spleen and prescriptions of Qinghaosu (Artemisinin) for treatment [7–10]. The Greeks also linked malaria with swampy and marshy environments [1, 9]. Further observed cases of tertian and quartan fevers reported by Hippocrates (460–370 BC), Celsius and Galen were high during the late summer and autumn seasons and coincided with the appearance of Sirius – the dog star illustrated by Homer in his epic [5, 9, 11, 12]. Gleaning from the Greeks, theories of Miasma emerged, with the Romans also attributing the disease transmission to either drinking marsh water or another positing the cause to be due to inhaled vapours from marshes [1, 9, 13]. During these periods Robert Koch introduced the Koch postulate in the era of the germ theory. This led to a new wave of infectious disease research meant to identify and relate pathogens such as the malaria parasites to the respective diseases.

Leveraging on the advances made in infectious disease research and the observation that there were other low malaria transmission areas with no swamp or marsh, Laveran observed and described the malaria parasite from the blood of malaria patients in Algeria in 1880 [14–16]. His initial finding was first met with scepticism not until 1896 when he further produced inevitable proof, which corrected the erroneous competing ideas of Ettore Marchiafava and Augusto Celli's attribution of the disease to a bacterium they named *Bacillus malaria* [13, 14, 17]. Entry of *Plasmodium* species into humans now became a subject of interest with the initial hypothesis suggesting a possible involvement of Mosquitoes as mooted by Sir Patrick Manson.

Inspired by the work of Laveran and Manson, Ross started his work to prove Manson's hypothesis at Secunderabad in India [18–20]. When confronted with a shortage of experimental samples after his posting to Calcutta, Ross resorted to the use of avian malaria models for his studies and this largely gave clues to his discovery [14, 19, 20]. Ross in 1897 established the involvement of mosquitoes in the transmission cycle and described the sexual developmental stages of *Plasmodium* within the mosquito [18–20]. This disproved the aspect of Manson's hypothesis, which suggested that the parasite-infected via an oral route through drinking water infested with the malaria parasite. It further disproved the postulate that transmission was by inhaling bad air from swamps [14, 19, 20]. Grassi and co-workers in 1898 also established that human malaria is solely transmitted by the female *Anopheles* mosquito [18–20]. Building on Ross' publications the team characterised the mosquito species (*Anopheles claviger*), which was involved in the transfer of the *Plasmodium* parasite and also described the sporogony stage of human *Plasmodium* [18–20].

#### **2. Global malaria trends in pre- and post-COVID era**

Past accounts of malaria and its transmissions were dealt with on a local basis and the clarity we have today was a bit blurred in those times. Increased knowledge of malaria and a much-coordinated global effort garnered in the battle against malaria within the early 1900s are yielding fruit today. However, this global effort has been met with various successes and challenges, that have reshaped previous strategies designed to meet milestone targets set from the period of millennium development goals (MDG) through to the current sustainable development goals (SDG). The success of the new wave of revolution against malaria is the disease eliminated in Europe and certain regions of the world as well as a massive decline in deaths due to the disease. In 2021, COVID-19 was estimated to have accounted for two-thirds of the challenges that were raised to have affected targeted milestones for malaria making it important to review the period in question.

Considering the global report on malaria from 2019 to 2022, the burden of malaria is still high showing an increase of 16 million more malaria cases reported in the post-COVID era than in the pre-COVID period (2018) [21]. A global outlook on malaria

#### *One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

case fatality rate (CFR) shows a moderately stable CFR of between 0.0% and 0.3% during the pre- and post-COVID period as illustrated in **Figure 1** for all the WHO regions. The trend further reflects a marginal decline in mortality from 625,000 in 2020 to 619,000 in 2021 [21]. Despite the slight decline in deaths for 2022, the numbers were still approximately 9% higher than malaria deaths reported during the pre-COVID period in 2018 (567,000) [21].

Malaria milestone targets set in 2015 are almost midway through the end date [21]. These targets were not entirely met by most of the WHO-endemic regional blocs with the exception of the South East Asia region, which was resilient through the period under purview. The worst-case scenario was observed for the Africa region, which accounted for 94% of the global cases of malaria in 2021 with an average estimated 0.3% case fatality rate (**Figure 1**). Of the mortalities reported in 2021 within the Africa bloc, 80% occurred in children under the age of 5 with Case incidence (cases per 1000 population at risk) and mortality targets being 45 and 75% off track respectively to the baseline target set for the Global technical strategy for malaria (GTS) for 2020 [21]. South East Asia region with most of its cases reported from India (79%) and the Eastern Mediterranean regions accounted for 2.5% each of the global cases of malaria. Western Pacific and American regions also contributed 0.6 and 0.2%, respectively (**Figure 1**) [21].

The recent COVID-19 pandemic in 2019, posited to have a zoonotic origin led to the enforcement of various restrictions. This did not only affect global commerce and international travel activities but also impacted health services and deliveries of which malaria is a part. Amid existing robust strategies in place for the elimination of malaria, COVID-19 has demonstrated the potency of potential future zoonotic pandemics to impinge on healthcare delivery and malaria milestone targets towards 2030. The panacea is to expand the strategic scope to include animal and environmental health mitigation strategies, which will demand open discussions and improve upon resilience in the fight against malaria.

#### **Figure 1.**

*Malaria burden in pre- and post-COVID period across WHO regions (author construct: 2023).*

#### **3. Human malaria transmissions**

Early studies on malaria have identified various forms of malaria and possible routes of entry used by the *Plasmodium* parasite to invade the host. This includes asymptomatic forms, congenital and neonatal malaria, cerebral malaria and imported malaria cases. These can be transmitted directly by a female *Anopheles* species or through parenteral transmissions including those acquired through blood transfusion, pricks from sharps and organ transplantation routes. Malaria infections irrespective of the route of entry in humans have been associated with unique cyclical fevers which were known from the past. These fevers could be described as tertian, quartan, and quotidian [5, 22–27].

Each of these fevers, with their severity and other unique clinical presentations, has been well characterized and known today to be associated with specific *Plasmodium* species [5, 23, 27–30]. Four main human *Plasmodium* species and a zoonotic species *Plasmodium knowlesi* are currently known to cause the disease in humans [5, 23, 27–30]. The identification of these *Plasmodium* species and their related fevers was therefore a remarkable breakthrough that has aided our better understanding of human malaria transmission today. While *P. knowlesi*, is currently the only known parasite species associated with quotidian malaria parasite-related fevers [5, 23, 27–30]. Tertian malaria on the other hand, which may exist in benign or malignant forms are known to be the most abundant indicating a high host and vector preference for members (*Plasmodium* species) of the tertian group [5, 23, 27–30].

#### **4. Non-human malaria transmission**

Non-human *Plasmodium* infections have played a significant role in unravelling the puzzle of human malaria transmissions. Non-human infections can be transmitted directly through the bite of a vector or congenital means to a non-human susceptible species. Laboratory transmissions to non-human vertebrate models are sustained and made possible parenterally using cell cultures or blood infected with the parasite [26, 31–34]. Moreover, cell-to-cell based transmissions are also made possible in cell cultures with studies investigating parasite development, drug resistance, host response, and parasite-host interactions benefiting immensely from such culture-based and animal-based model transmissions [26, 31–34]. Currently, more than 250 species of *Plasmodium* parasites are known today to infect various vertebrate hosts, including humans, non-human primates (Prosimian, Apes, old and new world monkeys), birds, reptiles, rodents and ungulates among other mammals [31, 35].

The use of transmission models or other vertebrate models, therefore serves as an alternative for complementary studies on confounding factors limiting malaria experimentations in humans [31, 34]. Ross and other independent workers pioneered research on avian malaria in 1897, this led to the discovery of various avian-specific *Plasmodium* species that are useful for laboratory studies [18–20]. This included studies on malaria transmission in different bird species and the identification of avian-specific vectors that are competent in the transfer of different *Plasmodium* species across susceptible bird species [19, 36]. The avian group served as a forerunner host model to unveil the sporogonic developmental stages of human malaria parasites within the mosquito vector.

Rodent malaria parasites, after their discovery in 1940, have also played a significant role in our current knowledge of malaria transmission over time [32, 37, 38]. This spans from basic laboratory research to iterative ones in both immunological and

#### *One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

malaria vaccine candidate tests. The rodent *Plasmodium P. berghei ANKA* has been found useful as a model for learning about cerebral malaria. Rodents are smaller in size requiring less housing space with a faster perpetuation time and like their bird counterparts they produce more offspring than most primates. Laboratory adaptable strains of rodent *Plasmodium* transmissions offer a comparative evaluation of malaria within a small controlled environmental condition.

*Plasmodium* transmission patterns among primates are similar to human malaria infections. Gleaning from the genetic relatedness of humans with great apes and other members of the monkey family, Laveran became the first to observe *Plasmodium pitheci* in the blood of orangutans in 1905. Further works on great apes in Africa in 1917 by Eduard Reichenow showed human-like *Plasmodium* parasites in the blood of captive chimpanzees and gorillas in Cameroon. Following Reichenow's observations, Blacklock and Adler in 1922 also described and confirmed *Plasmodium* parasites resembling *P. falciparum* in chimpanzees in Sierra Leone [25, 39, 40]. On the other hand, similar transmissions are also observed in the old and new-world monkeys. *P. cynomolgi* happens to be the first monkey *Plasmodium* species detected and characterised by Mayer in 1907 from a *Macaca cynomolgus* monkey [23, 41]. Further, observations by Robert Koch in Monkeys of East Africa in 1947, also served as the basis for discovering the third malaria cycle in primates [23, 28]. The work of Shortt and Garnham also further described malaria relapse or recrudescence in *P. cynomolgi* in 1948. Krotoski and co-workers also described the hypnozoite stages of the parasite [5, 23, 30, 42, 43].

#### **5. Emerging zoonotic malaria and transmission concerns**

Zoonotic malaria is an emerging form of malaria that affects and is shared by both humans and other mammals. Cross-species malaria transmissions have been observed with certain groups of *Plasmodium* species transmitted among different mammalian host species in nature and experimental settings. This raises concerns that borders on the need for one-health and it also indicates the possibility for susceptible mammals to double as reservoirs and hosts for their host-specific and non-specific *Plasmodium* species, which are competent enough to cause the disease across these mammal species. Secondly, frequent misdiagnosis of non-human *Plasmodium* species of malaria that infect humans is also a contributory factor. The continuous transmissions are also posited to have a great impact on human malaria eradication pegged for 2030 and the conservation of susceptible and highly endangered mammal species [31]. There are also knowledge gaps that are a source of concern and will require continuous and active research to ascertain the posited impact of this form of the disease. This will help develop effective mitigating actions to manage or deal entirely with such malaria transmissions.

Frequent passages of malaria parasite cell lines under laboratory conditions for certain lower mammal and vertebrate species have been successful. This has been adopted in the generation of human adaptable strains and mutants of *P. knowlesi*, *P. berghei*, *P. falciparum mutants 3D7 and Hb3* among others that are used for various malaria *in-vitro* studies [32, 37, 44–47]. The adaptability of such malaria parasite species to subsist and thrive on human red blood cells or non-human host red blood cells within cell cultures is an interesting observation to partly explain the multiple switch postulate, which is believed to account for the evolution of malaria parasites [48, 49]. This suggests that *Plasmodium* species has evolved with a high tendency to switch to

different red blood cells among mammalian species. Though there are currently no known *Plasmodium* parasites of the lower mammalian species that infect humans in nature since culturing conditions are different from intact physiological conditions, however, it is envisaged that when given factors that frequently promote such contacts continue, human-adapted malaria parasite strains may evolve and circulate. Promoting factors such as laboratory spillovers, the presence of competent transmission vector, environmental conditions and host factors that bring proximity to the human-*Plasmodium* mutant interphase should be carefully investigated, and managed appropriately with this possibility in mind.

Non-human primate *Plasmodium* species are naturally and experimentally transmissible to humans and vice versa [31, 50–52]. Such cross-species infections are an emerging area of concern to current malaria transmission patterns facilitated by increased invasion, contact, and proximity to shared forest environments with non-human primates. Furthermore, host primates and vector confinement to certain geographic regions of the globe also influence the kind of circulating *Plasmodium* pathogen that can be transmitted [31, 35, 40]. Infections with *P. knowlesi* a type of monkey malaria, was the first zoonotic malaria to be described in 1965 to naturally infect humans [27–29, 39, 53]. Such infections are common within South East Asian countries where it is transmitted mainly by the vector *Anopheles balabacensis* [54]. *P. knowlesi* infections have been the main cause of malaria in Brunei and Singapore. In Malaysia, of the 2607 malaria cases reported in 2020, all were due to *P. knowlesi* infections with no report of any of the known human *Plasmodium* parasites. Malaria trends in Sabah over 5 years from 2014 to 2018 reveal that *P. knowlesi* over the period, accounted proportionally for 0.78 (13,569/17310) of all malaria cases which were reported [54].

Cases of fatal and mild forms of the disease due to *P. knowlesi* infections in humans have been reported in endemic communities, but the true prevalence is still not known [28, 55, 56]. Diagnosis using PCR aids in effective speciation and detection but the technique is not easily applicable in remote areas where the main host vertebrate *Macaca fascicularis* and human contacts are high. Methods such as microscopy are elusive due to their morphological resemblance to human *Plasmodium* parasites. Furthermore, whether current human infections are through the human-mosquitohuman route or the Macaque-mosquito-human route, is not yet clear. Potential for natural human infections and experimental infection has also been demonstrated for other non-human primate *Plasmodium* species such as *P. cynomolgi, P. inui*, *P. brasilianum*, *P. coatneyi* and *P. simiovale* [57]. Putaporntip and coworkers after testing 1359 febrile human blood samples from 2007 to 2018 detected 9 cases of *P. cynomolgi* coinfections within 5 provinces in Thailand [58]. Further works by Yap and coworkers in 2021 also led to the detection of infections with *P. coatneyi (3), P. cynomolgi (9)* and *P. inui (3)* among humans [59].

Apes are the main host of *Plasmodium* representatives of *Laverania* except for *P. falciparum,* which is a human *Plasmodium* pathogen [27, 60–62]. While Orangutans and Gibbons are found in Asia, Chimpanzees, Bonobos and Gorillas who are humans' closest relatives are found in Africa [39, 53, 60, 63, 64]. Such host and vector distribution patterns are important to note as they affect the disease transmission dynamics. Special *Plasmodium* species groups are known to uniquely infect the different Ape hosts of which most share a resemblance with the human *Plasmodium* versions. Besides these observations, *P. rodhain* and *P. schwetzi* infection in humans has been demonstrated and shown to be successful through blood inoculations [5, 23, 30, 62, 65]. Infections of Chimpanzees with *Plasmodium vivax* and *P. malariae* have also been

#### *One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

demonstrated experimentally [60, 64, 66]. Another investigation into the prevalence of simian malaria identified *P. falciparum,* which was earlier thought to be a humanspecific species for malaria infections to have naturally infected *Macaca radiata* and *Macaca mulatta* in India [51]. Suggesting that both humans and the great Apes could serve as reservoirs and hosts for human *Plasmodium* species and Ape *Plasmodium* parasite species. The consequence of reverse zoonosis is likely to evolve new and more virulent or attenuate existing species, which will likely change host and pathogen infection and transmission dynamics.

#### **6. Path to the development of malaria**

The establishment of the *Plasmodium* parasite in the blood of humans, non-human primates and other vertebrate hosts is important for malaria to occur [26, 67, 68]. This is mediated through a meal bite from a transmissible competent mosquito vector species harbouring infective stages of the parasite, this is important for malaria to occur as illustrated in **Figure 2** [69]. *Plasmodium* sporozoites after introduction into the host by the mosquito, invade hepatocytes and develop into merozoites, which further invade erythrocytes and develop into schizonts and gametocytes. For further perpetuation, gametocytes of *Plasmodium* are fed on by a mosquito and *Plasmodium* completes the sexual stages of their development to form asexual sporozoites and the cycle continues with another blood meal as illustrated below [69].

Clinical presentations of human malaria are well characterised and mainly known to be associated with cyclical fevers due to the continuous invasion, infection and re-infection of hepatocytes and erythrocytes by various stages of *Plasmodium* and the response reactions from the immune system of the host. In the uncomplicated scenario, the disease may be accompanied by chills, nausea, vomiting and headaches. Complicated malaria, which is a more severe form of the disease characterised by

cerebral malaria with its accompanying seizures, severe malaria anaemia, coma, metabolic acidosis and death also, do occur. This is often seen among vulnerable groups such as children under 5 years of age, immune-compromised persons and persons with no pre-existing immunity. Furthermore, there is also malaria relapse caused by hypnozoites, which is associated with recurrent and difficult-to-treat malaria infections that may linger on for years.

#### **6.1 Parasite development in vector**

Mosquitoes become infected when they ingest human blood containing gametocytes. These gametocytes differentiate into male and female gametes within the mosquito's gut, facilitated by the mosquito's blood meal. Following fertilization, the zygote develops into a motile ookinete within 24 hours of having imbibed a blood meal. This extracellular ookinete then departs from the blood mass, navigates through the midgut epithelium and evolves into a growing oocyst in the basal subepithelial region located between the midgut epithelium and the basal lamina (BL) [70–72]. The midgut's peritrophic membrane (PM) functions as an initial defence line in *Anopheles* mosquitoes against ookinetes, providing a physical barrier [73]. Ookinetes release the chitinase enzyme to assist in navigating through the PM [74]. Similarly, ookinetes are confronted with midgut proteases. To counteract these proteases, ookinetes produce surface proteins called P25, P28 and P47, pivotal for their successful invasion of the midgut [75–77].

Several factors, such as the vector's type, parasite strain, environmental circumstances and host immunity, influence the development of the malaria parasite inside the vector. Vector competence refers to the inherent capacity of anopheline species or populations to facilitate the progression of *Plasmodium* parasites, transforming from ookinete to infective sporozoite [78]. Variation in vector competence occurs among different vectors and parasite combinations, and this variability can be influenced by genetic, molecular, immunological and ecological factors [78–81]. For instance, particular genes within mosquitoes can impact parasite infections, either amplifying or constraining their growth [81, 82]. Similarly, specific molecules produced by parasites can engage with the mosquito's immune system or midgut microbiota, enabling evasion or overcoming of the vector's defences [83, 84]. Additionally, environmental elements such as temperature, humidity, rainfall and seasonal variations can shape vector competence by influencing mosquito physiology, behaviour and lifespan [81, 84–86].

Managing malaria faces a significant challenge in light of the rise and spread of insecticide resistance within *Anopheles* mosquitoes. The emergence of resistance to insecticides can undermine the efficacy of strategies such as insecticide-treated nets (ITNs) and indoor residual spraying (IRS), which are designed to minimise human-mosquito interactions. While the precise mechanisms and outcomes are not entirely clear, insecticide resistance can also affect the abilities of malaria vectors, as indicated by a study [78]. Several studies have suggested that the impact of insecticide resistance on vector competence can fluctuate, depending on variables like the type of insecticide, the mechanism of resistance, the species of mosquito and the strain of the parasite. For instance, certain research has revealed that the rates of *Plasmodium* infection in pyrethroid-resistant mosquitoes can either increase or decrease compared to susceptible counterparts [78]. Furthermore, investigations have unveiled that insecticide resistance can have an impact on diverse facets of mosquito behaviour and fitness, including elements such as reproductive capacity, survival, feeding patterns and host preference. These factors, in turn, can shape the vectorial capacity,

a measure of the potential for a mosquito population to transmit malaria [78]. As a result, it becomes crucial to observe and control insecticide resistance in malaria vectors, while simultaneously devising new methods and strategies to prevent or decrease the transmission of malaria.

#### **6.2** *Plasmodium* **parasites of malaria**

*Plasmodium* species that cause malaria among vertebrates are diverse. The species responsible for causing malaria in humans are in five distinct forms: *Plasmodium falciparum*, *Plasmodium vivax*, *Plasmodium malariae*, *Plasmodium ovale sensu lato* (s.l.), which encompasses *P. o. curtisi and P. o. wallikeri* and *Plasmodium knowlesi* [55, 87]. Human *Plasmodium* species originate from different evolutionary lineages, sharing common ancestors with nonhuman primate species [25, 61, 88]. As anticipated, the five primary malaria-causing parasites exhibit distinct biological characteristics throughout their respective life cycles [5, 89].

One pivotal stage in this life cycle is the invasion of human red blood cells by *Plasmodium* species. While *P. falciparum* shows no particular preference for a specific age of red blood cells, *P. vivax* and the two species within *Plasmodium ovale sensu lato* (s.l.) tend to infect young red blood cells or reticulocytes. Conversely, *Plasmodium malariae* is thought to invade older red blood cells [90, 91]. Differences are also apparent in the timing and lifespan of gametocyte production, both of which are critical components of fitness [92, 93]. *Plasmodium falciparum* undergoes a more extended maturation process, spanning five stages over 9–12 days, and remains infectious for several days compared to other malaria parasites. In contrast, *Plasmodium vivax* and *P. ovale s.l.* have dormant stages or hypnozoites that lead to relapses after the initial infection; these stages are absent in *P. falciparum* and *P. malariae* [42, 94]. Another notable distinction is the ability of *P. falciparum*-infected red blood cells to adhere to the endothelium of capillaries and venules, a phenomenon called sequestration, which is associated with severe clinical symptoms uncommon in non-falciparum malaria [95, 96]. These traits represent only a fraction of the many variations among *Plasmodium* species causing malaria in humans.

In addition to *Plasmodium* species that primarily affect humans, there are zoonotic malaria parasites originating from nonhuman primates. The most prominent example is *Plasmodium knowlesi* found in macaques across Southeast Asia [55, 97–99]. However, evidence suggests that other nonhuman primate malaria species also function as zoonotic agents in this region, sometimes causing asymptomatic infections [99–101]. Moreover, anthropozoonotic malaria parasite cycles involving nonhuman primates in South America have been documented [41, 102, 103]. Despite the focus on zoonotic species, there are at least 39 known *Plasmodium* species affecting nonhuman primates globally, encompassing both formally described species and detected lineages [26, 67]. Some of these parasites received limited attention until recently, potentially due to challenges associated with studying nonhuman primates. Additionally, there was a significant emphasis on viable animal models to enhance the understanding of malaria biology and to explore treatment or vaccine options. Regardless of the factors that hindered their study, substantial progress has been made in the past two decades.

#### **6.3 Biology and diversity of malaria vectors**

Malaria is a severe and occasionally fatal illness caused by a parasite that infects specific mosquito species, which subsequently bite humans. The characteristics and variety of malaria carriers play a pivotal role in influencing malaria transmission and control. These carriers, belonging to the *Anopheles* genus, consist of over 400 species, with approximately 70 of them recognised as transmitters of malaria to humans [104]. Non-Primate transmissions are also facilitated by Mosquito vectors such as Aedes and Culex species.

The diversity of malaria vectors varies across different regions and habitats, contingent on environmental and climatic factors that impact their breeding, feeding and resting habits. Among the significant malaria carriers are *Anopheles gambiae sensu lato (s.l.), Anopheles funestus s.l., Anopheles arabiensis, Anopheles stephensi, Anopheles dirus, Anopheles minimus, Anopheles farauti, Anopheles darlingi* and *Anopheles albimanus* [104]. These vectors exhibit different preferences for host species, biting times, indoor or outdoor locations and larval habitats. For instance, *A. gambiae s.l.* is highly anthropophilic (prefers to feed on humans) and endophagic (feeds indoors), while *A. arabiensis* is more zoophilic (prefers to feed on animals) and exophagic (feeds outdoors) [105].

The genetic structure of malaria vectors' populations constitutes a pivotal facet of their biology and diversity, serving as a reflection of their evolutionary history, adaptability, gene flow and reproductive isolation. Population genetics aids in tracing the origin, distribution and movement of these vectors, as well as their susceptibility or resistance to insecticides and malaria parasites. Molecular methods like polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP), microsatellite analysis and DNA sequencing enable the assessment of genetic diversity and population structure of malaria vectors [106]. For example, PCR can distinguish the members of the *A. gambiae* complex, a group of morphologically similar sibling species differing in their vector capacities.

Comprehending the biology and diversity of malaria vectors is vital for grasping the epidemiology and ecology of malaria transmission, as well as for devising effective strategies for vector control. With knowledge about the attributes and behaviours of diverse malaria vectors in various settings, targeted interventions such as insecticide-treated nets (ITNs), indoor residual spraying (IRS), larval source management (LSM) and genetic modification (GM) can be employed [107]. However, the biology and diversity of malaria vectors are dynamic and influenced by factors like environmental shifts, human activities, insecticide resistance and parasite evolution. Consequently, ongoing monitoring and surveillance of malaria vectors remain indispensable for keeping information up to date and adapting interventions accordingly [107, 108].

#### **7. Transgenic mosquitoes**

Transgenic mosquitoes could be a successful choice for developing a range of effective disease-fighting strategies as a result of the advancements made in genetic modification in recent years [109–111]. Arthropod control by genetic modification is an old concept that was independently researched in the 1930s, 1940s and 1950s by three pioneers: A. S. Serebrovskii (1940), F. L. Vanderplank (1947) and E. F. Knipling (1955) [112, 113]. Furthermore, the 1970s saw a particular focus on studies concerning sterile males [109, 110, 114], which in turn prompted re-evaluation of the original concepts [109–111, 115–118].

The genetic modification of mosquitoes for the prevention of malaria is primarily focused on two main strategies: population suppression, which aims to eradicate or

#### *One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

drastically reduce mosquito populations; and population modification, which aims to render the natural population immune to infection by expressing anti-plasmodial agents or altering mosquito genes crucial for parasite transmission [119–121]. Significant progress has been made in developing transgenic mosquitoes that are resistant to the parasites that cause malaria over the past 10 years. These developments include changes to mosquito genes essential for parasite development [122, 123] enhancement of mosquito immune components [124–126], the introduction of antiparasitic toxins or molecules [125, 127–129] and the expression of single-chain antibodies targeting parasite proteins [130, 131]. The creation of the first genetically modified mosquito in Africa through the Transmission Zero initiative holds special importance. This global scientific program is led by researchers from Imperial College London and the Ifakara Health Institute (IHI) in Tanzania, in collaboration with the Tanzanian National Institute of Medical Research (NIMR). This mosquito strain has been subject to genetic alterations that will ultimately provide researchers with the ability to hinder malaria transmission by mosquitoes. This noteworthy scientific achievement represents a pivotal advancement in the renewed worldwide efforts to eradicate malaria from the African continent [132].

The generation of transgenic mosquitoes poses a significant challenge, involving issues related to regulations, ethics and social aspects linked with the introduction of genetically modified organisms into the natural environment [133]. The transition of this approach from controlled laboratory settings to field trials in disease-endemic countries (DECs) is a gradual process aimed at maximising potential epidemiological benefits while minimising potential complications during implementation [134]. Among the multitude of factors to be taken into account is the irreversible nature of releasing organisms, as once released, it becomes exceedingly challenging to retrieve them in case issues emerge [133]. Unlike traditional methods of vector control, the utilisation of transgenic mosquitoes will encounter varying expectations from different stakeholders. Effective management of these divergent expectations is crucial to ensure ongoing support and mitigate opposition. This management is essential for a comprehensive assessment of potential public health benefits [134].

#### **8. Environmental impact of** *Plasmodium* **transmission**

Intriguingly the *Plasmodium* parasite sits on a knife edge as it will have to navigate different and mandatory environmental landscapes in a bid to survive and procreate. Traversing through the internal environment of a single or more than one vertebrate host or reservoir, the parasite is presented with a myriad of challenges including the need to usurp, manage or adapt to the varying degree of immunity presented by the host or reservoir as well as pressure from antimalarials if in use by the host. Moreover, co-infections be it with other *Plasmodium* species or other pathogens are also common and unique and may also influence susceptibility patterns of the host vertebrate to invasion and subsequently disease manifestation or an asymptomatic state. Host or reservoir interactions may be facilitated after entry of the pathogen via conduits such as a competent vector direct to the host, indirectly from the mother-to-child transmission and blood transfusion routes. The journey back to complete the sexual phase of the pathogen's development is precarious as it is greatly dependent on the survivability and existence of a competent vector. Vectoral characteristic consists of sex, feeding behaviour and time and state of internal and external parameters among others which are paramount to

sustain the vector role to maintain the sexual stages of the *Plasmodium* parasite for further development and later transmission.

External environmental factors do influence the survival and the persistence of vectors and subsequently, enhance the transmission of their respective *Plasmodium* species. Factors promoting transmissions may include seasonal changes with a high prevalence of malaria cases and deaths linked to the wet season in most of Africa [107, 135, 136]. Mosquito populations and bites increase due to enabling conditions that facilitate the breeding of the vector throughout the wet period and summer [57, 105]. Another important factor is temperature, most of the vector species reduce their bites and also have their development from larvae to adult stage extended when temperatures are cold [57, 67, 105]. Changes in land use such as for Agriculture and irrigation, also provide breeding grounds for mosquitoes and a cool hiding place among growing plants for adult mosquitoes [137]. Invasion of the forest environment either legally for the construction of public goods like bridges and roads or illegally for hunting and exploitation of natural resources such as mining of minerals are on the increase, this exposes such workers to non-human primate mosquitoes [61, 137]. Uncovered created pools of water may serve as breeding grounds for mosquitoes. Fragmentation and indiscriminate or uncontrolled degradation of forests due to logging or the creation of human community settlements deplete forest tiers and also increase human exposure to non-human primate mosquitoes and *Plasmodium* species [137, 138].

Control and preventive programs for the external environment were designed to target mosquito vectors at all stages of development. Insecticide use, use of insecticide-treated bed nets, use of mosquito repellents, draining of swamps and puddles as well as wearing of protective garments that obscure the skin among others are used to exterminate mosquito populations or avoid their bites. Most of such earlier malaria control and elimination campaigns that adopted these strategies were not globally coordinated and though they made some achievements, they could not eliminate malaria or sustain the momentum. The use of chemicals such as DDT came along with its concomitant impact on human health and non-targeted fauna within the environment leading to disturbance in ecosystem balance and this has increased the dominance of some invasive species. The development of resistant mosquitoes to insecticides due to frequent usage has also been observed. Furthermore, DDT and organophosphate usage among other chemicals in Australia, have been shown to contribute to bird poisoning which is often characterized by birds falling from the skies [139]. This reveals the potential effect of DDT on humans through the food chain and other environmental conduits [139].

#### **9. Ending malaria transmissions by 2030 the one-health way**

Eradicating malaria by 2030 from the human population is achievable given the current strategies and coordinated momentum. The close observation of benchmarks and prompt interventions are robust. However, an entire win against malaria transmissions will demand a one-health approach to deal with possible re-invasion of *Plasmodium* and their respective vectors from animal reservoirs. This is a form of systems thinking that tackles at the same time, health matters with a human, animal and environmental outlook. Acting as a paradigm shift from the past approach of independent mono-health perspective adopted. Human health has always been the main agenda, ensuring a healthy environment will therefore demand a fair balance

#### *One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

in already existing malaria control and elimination intervention programs. This will require the use of eco-health strategies involving environmental impact and risk assessment, which puts into perspective the consequence of interventions on the health of humans, animals, plants and other components of the environment. On one hand, there will be the need to improve surveillance of non-human primate malaria among non-human primates and humans. Enacting and enforcing conservation laws using strategies such as restoration, maintenance and regulation of deforestation will also be required to control the level of human invasions.

Mosquitoes are hematophagous and have been successful over many years serving as vectors for the transmission of malaria and other infectious diseases. The induction of selective pressures for the mosquito vector from transgenes or by-products of a poorly introduced control or preventive intervention may have dire consequences on malaria transmission and other mosquito-borne diseases [79, 123, 129, 140, 141]. Mosquito vectors under genetic pressures may select new preferences and competency for hosts, bridge and or transmit new infectious disease pathogens across the human-animal interphase. Healthy balance for ecosystems shared with mosquitoes during the introduction of vector control interventions will therefore be required at all times. It is the surest way to contribute to a sound and healthy world for all species involved. This will ensure that potential invasive organisms may not take the opportunity of the tilted balance to proliferate and eliminate other species from extinction. A well-coordinated research effort and shared data for humans, animals and the environment will be required. The recent transgenic mosquito trials ongoing in Tanzania are among the most wonderful achievements of the millennium with a promising outcome to contribute together with other adopted arsenals for eliminating malaria [124, 134]. This will require a careful evaluation of such programs with one health lens in order to be able to anticipate possible impacts on the environment and other species.

#### **10. Conclusions**

Fundamental to our understanding of malaria transmission today would have lingered on evasively for many years with its massive toll on human life except for the timely merger of past and current coordinated knowledge. Past information on malaria though was sparse and scattered; it still, resonates with the concept of one health which is a current paradigm for the achievement of holistic health. Drawing from careful observation of patient records, the unique cyclical nature of each *Plasmodium*-specific fever and environmental events associated with changes in seasonal patterns; land use, and hydrological distribution patterns. Though the level of knowledge in certain cases was not entirely accurate in the past, it paved the way for the discovery of medications for the disease, initiation and adoption of control and preventive interventions that caused a decline in malaria and are also very useful today. The corroborative involvement of the study of other vertebrate species was remarkable in unearthing some unsolved puzzles about the disease cycle and other aspects of malaria research. Zoonotic malaria is currently an emerging challenge and possibly ongoing within the African region where there are great Apes and also in South America. Drawing from the current widespread dominance of *P. knowlesi* malaria in Malaysia and some Southeast Asian countries, zoonotic malaria will require maximum attention to evaluate the changing dynamics in malaria transmission in these regions that bear the biggest burden of the disease. Past concentration

skewed towards human-centred research and programs will require a one-health approach. These will include a well-coordinated approach involving human and animal programs, locale-specific friendly interventions and the environment at large to increase the tendency of achieving the malaria eradication target set for 2030 and beyond.

#### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Jonas Bedford Danquah\* and Jennifer Afua Afrifa Yamoah Council for Scientific and Industrial Research, Animal Research Institute, Accra, Ghana

\*Address all correspondence to: jonydan4@gmail.com

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*One Health Perspective of Malaria Transmission DOI: http://dx.doi.org/10.5772/intechopen.113908*

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#### **Chapter 4**

## Malaria Transmission Dynamics in East Africa

*Fred Anangwe Amimo*

#### **Abstract**

Malaria remains one of the deadliest mosquito-borne diseases in the world and indeed in sub-Saharan Africa and the sub-region of East Africa. The sub-region ranges from the coastal landscapes of Kenya and mainland Tanzania to the east borders of the congolian tropical rain forest and river basin on west Uganda boundary. The many water bodies in the region provide breeding grounds for *Anopheles* mosquitoes which transmit malaria affecting large populations of humans. Domestic animals and wildlife also play a pivotal role in malaria transmission by providing micro-breeding and resting sites via their footprints and sheds. The dynamics of transmission of malaria therefore include the presence and behaviors of the *Anopheles* vectors, the prevalence of the *Plasmodium* parasites, seasonality, climate change and related environmental factors favoring transmission in East Africa, and affected human hosts. Rainfall patterns and temperature stand out in affecting both the vector and malaria parasite life cycle. Inadequate use of preventive measures and treatment regimens has increased the risk of transmission of the parasites. This chapter explores the dynamics and trends of malaria transmission in this part of Sub-Saharan Africa.

**Keywords:** malaria transmission, *Anopheles* mosquitoes, East Africa, *Plasmodium* parasites, dynamics

#### **1. Introduction**

Malaria is still one of the deadliest mosquito-borne diseases in the world and has remained the cause of many infections and deaths in East Africa [1, 2]. There are approximately 3500 known species of mosquitoes worldwide. Among them, the genus of *Anopheles* mosquitoes transmits malaria in sub-Saharan Africa. This genus contains more than 460 recognized species, and approximately 60–100 *Anopheles* species have been implicated in transmitting malaria parasites [2, 3]. East Africa is indeed a sub-region of Sub-Saharan Africa and is endowed with a tropical climate, diverse landscapes, and remarkable ecological features that are conducive for spread of malaria. The sub-region encompasses a large area, extending from the coastal areas of Kenya and mainland Tanzania to the western borders of Uganda and east Congo River basin. The Great Rift Valley, a geological trench that traverses these countries, is composed of escarpments, volcanoes, lakes, and hot springs and a unique tropical biodiversity within which *Anopheles* mosquitoes flourish and spread malaria.

At the core of East Africa is Lake Victoria whose basin supports year round malaria transmission. The lake provides important freshwater resources and sustains the livelihoods of millions of people who due to exposure to malaria have diverted their economic development in terms of these resources needed for prevention and cure of the disease [4]. Other lakes in the sub-region which experience malaria transmission include Lake Tanganyika, Lake Malawi, and the numerous freshwater lakes located in the Great Rift Valley. Malaria thrives in such areas with stagnant water, which serve as breeding grounds for mosquitoes. The presence of these lakes and other fresh water bodies in lowlands and some highland areas of East Africa have contributed to the prevalence of malaria. The *Anopheles* mosquitoes concurrently find suitable habitats for breeding within their bounds [5].

#### **1.1** *Anopheles* **breeding and proximity to human populations**

*Anopheles* mosquitoes breed in relatively clean stagnant or slow-moving water bodies including water storage tanks, temporary pools, streams, river bank vegetation shelters, rice irrigation fields, ponds and swamps which are widespread in the sub-region [6]. In urban areas, *Anopheles* mosquitoes tend to breed in artificial water containers, such as jerricans, barrels, and discarded items that collect water. These breeding sites are often found in close proximity to human residences. Moreover, poorly maintained drainage systems and improper waste management have created stagnant water pools suitable for mosquito breeding in most of these urban areas. In rural areas *Anopheles* mosquitoes breed in natural water bodies like streams, river banks, ponds, and swamps. These breeding sites are often located in close proximity to human settlements, particularly in areas where water sources are shared between the inhabitants and breeding by mosquitoes [7]. A case in point is the village communities living in vicinity of rivers and areas with rice irrigation fields in which the paddies provide breeding grounds for *Anopheles* mosquitoes. The many rice irrigation schemes in the sub-region have attracted huge settlements where immune and nonimmune human populations provide labor in the fields and thereby come into close proximity of breeding sites of the *Anopheles* vectors of malaria.

Common human behaviors in East Africa such as storing water in containers or engaging in outdoor activities near water bodies creates additional breeding sites for mosquitoes unintentionally. Furthermore, settlement patterns in this region show a pattern of close proximity of houses to water bodies, which affects the distance between breeding sites and human populations. However, the specific distance between *Anopheles* mosquito breeding habitats and human populations differ widely and are determined by local environmental and socio-economic factors. The traditional east African type of house constructed in lowlands is mosquito permissive and contributes to massive entry of *Anopheles* mosquitoes into the houses mainly through the eyes and bite inhabitants without treated bed nets thereby increasing the chances of malaria infection.

#### **1.2 Role of domestic animals**

While humans are the primary hosts for malaria parasites, mosquitoes feed on both humans and animals, and some domestic animals have been found to contribute to the dynamics of malaria transmission in the region. Female *Anopheles* mosquitoes require blood meals to complete their reproductive cycle. Besides feeding on humans, they bite and feed on domestic animals, including cattle, goats, sheep, pigs and dogs.

#### *Malaria Transmission Dynamics in East Africa DOI: http://dx.doi.org/10.5772/intechopen.113192*

Domestic animals have indeed been implicated in influencing the abundance and density of *Anopheles* mosquitoes. Livestock, such as cattle and goats, provide blood meals for mosquitoes, which support their survival and reproduction. Areas of East Africa with higher livestock populations tend to experience increased mosquito densities, which subsequently elevate the risk of malaria transmission to both humans and animals [8]. Cattle require water for drinking, and their presence near water bodies have contributed to the creation of additional breeding habitats for *Anopheles* mosquitoes. Cattle troughs, footprints, watering holes, and areas where animals congregate providing suitable breeding sites for mosquitoes when the water becomes stagnant or poorly drained.

#### **1.3 Vegetation cover**

Vegetation, inclusive of forests, savannas, dense vegetation near water bodies, and maize plantations provide suitable breeding sites for *Anopheles* mosquitoes. These vegetation areas usually have natural depressions, puddles, whorled leaves or small pools of water that collect rainwater, creating stagnant water bodies where mosquitoes lay their eggs. High vegetation coverage near human settlements increases the likelihood of mosquito breeding sites in close proximity to human populations. Changes in vegetation coverage due to deforestation, land conversion for agriculture, and urbanization has contributed to mosquito breeding and malaria transmission. Deforestation, for example, has led to alterations in mosquito habitat availability, water availability, and human-animal interactions, potentially influencing the transmission of the disease [9]. The dynamics of the role of vegetation cover in *Anopheles* mosquito breeding depend on factors such as the prevalence of mosquito vector species, change in temperatures and variable rainfall patterns in the local habitats.

#### **2. Components of malaria transmission dynamics**

East Africa is known to have a high burden of malaria attributable to *Anopheles* mosquito vectors with several factors contributing to its transmission [10]. Malaria transmission dynamics in the sub-region is therefore characterized by the presence of the *Anopheles* mosquito vector species, the *Plasmodium* parasites that cause the disease, human host factors, and environmental factors including transmission seasonality and climate change [11].

#### **2.1** *Anopheles* **species and** *Plasmodium* **parasites**

The primary mosquito vectors responsible for malaria transmission in East Africa are mainly *Anopheles gambiae*, *An. arabiensis* and *An. funestus*. These mosquito species thrive in the region's diverse ecological backgrounds, including urban centers and rural areas of both lowland and highland settings. In Kenya, mainland Tanzania, and Uganda *An. gambiae and An. funestus* are the most efficient and widespread mosquito vectors. They breed in various water bodies, such as stagnant puddles, rice fields, irrigation channels, and river edges [11–13]. The most prevalent species of malaria parasites in the sub-region are *Plasmodium falciparum*, which is responsible for severe cases and most malaria-related deaths, and *Plasmodium vivax*, which is less common but still present in some areas. *Plasmodium falciparum* is the most deadly of all malaria parasites and is responsible for the majority of severe malaria cases and

malaria-related deaths in East Africa [14]. It is known for its ability to rapidly multiply in the bloodstream and cause severe complications, such as cerebral malaria and severe anemia. *Plasmodium vivax* has the unique ability to form dormant liver stages, the hypnozoites, which can lead to recurrent infections even after successful treatment of the acute phase [15] thus propagating transmission further.

#### **2.2 Human host factors**

The risk of malaria infection is influenced by various human factors. These include individual immune responses, genetic factors, previous exposure to malaria, their behavior, nutritional status and travel among others. Vulnerable community members pertaining to severe malaria include young children, pregnant women, non-immune individuals and those with weakened immunity probably from malnutrition common in the region. With acquired immunity, those in the region living in endemic areas have developed partial immunity over time through constant repeated exposure to *Plasmodium* parasites. Among those living in endemic regions such as around Lake Victoria are cases of genetic variants of sickle cell trait that are less vulnerable to malaria infection. Behaviorally, there is a tendency for some people who live in areas with high mosquito density and have not embraced usage of repellents and mosquito nets risking being bitten by infected mosquitoes. With much poorly constructed homes and limited access to health facilities for diagnosis and treatment, malaria has remained a significant public health challenge in East Africa, leading to substantial morbidity and mortality, particularly among pregnant women and children under 5 years of age [12].

#### **2.3 Transmission seasons and suitable conditions**

East Africa's diverse landscapes, from coastal regions to high-altitude areas, play a significant role in malaria transmission. Mosquito breeding sites, such as stagnant water bodies, are abundant, and suitable climatic conditions support the survival and reproduction of both mosquitoes and parasites. Kenya's diverse climate, characterized by distinct wet and dry seasons, significantly influences malaria transmission patterns. In all other major East African Countries, there is an increase in mosquito breeding sites due to the accumulation of water during the rainy season. The high humidity and temperatures during this period accelerate mosquito development and the incubation of malaria parasites within the mosquitoes [16–18]. Malaria transmission in the region is seasonal, with peak transmission occurring during and after the rainy seasons when mosquito breeding sites increase. This seasonality leads to fluctuations in malaria incidence, with higher transmission rates during the wetter months. The availability of suitable breeding sites, especially during the rainy season, facilitates the rapid proliferation of these mosquito populations, leading to increased malaria transmission.

#### **3. Underlying factors in malaria transmission**

#### **3.1 Malaria vector presence**

In East Africa, several *Anopheles* mosquito species are known to be important vectors of malaria. The predominant species vary in different regions and over time due to various factors, including environmental conditions and human interventions. *Malaria Transmission Dynamics in East Africa DOI: http://dx.doi.org/10.5772/intechopen.113192*

Precisely, the *Anopheles* mosquito species commonly associated with malaria transmission in East Africa are *Anopheles gambiae* complex and *Anopheles funestus*. The *Anopheles gambiae* complex contains multiple sibling species, including *Anopheles gambiae* and *Anopheles arabiensis* [19]. *Anopheles gambiae* is often the most important malaria vector in many parts of East Africa, while *Anopheles arabiensis* is also a significant contributor to malaria transmission. *Anopheles funestus* is found in various parts of East Africa and also plays a significant role in malaria transmission. The relative abundance and distribution of these species varies across different areas of the sub-region and may also change over time.

#### **3.2 Distribution and density of** *Anopheles* **mosquitoes**

The distribution of the various *Anopheles* mosquito species which are associated with malaria transmission in East African countries of Kenya, mainland Tanzania and Uganda include *Anopheles gambiae*, which is widespread in sub-Saharan Africa. Other countries in sub-Saharan Africa where *An. gambiae* is found are Nigeria, Ghana, Burkina Faso, Cameroon, and Sudan among others. *Anopheles funestus* is found in widespread areas of East Africa and extends further south in countries such as Mozambique, Malawi, Zambia, Zimbabwe, and parts of South Africa. *Anopheles arabiensis* is prevalent in sub-Saharan Africa, and includes Kenya and mainland Tanzania among others such as Ethiopia, Sudan, and Somalia. It is particularly known for its adaptability to arid and semi-arid environments. *Anopheles merus* is primarily found in coastal areas of East Africa, including parts of Kenya and mainland Tanzania. *Anopheles coustani* is not only found in East Africa but also parts of West and Central Africa. *Anopheles rivulorum,* a closely related species to *Anopheles funestus* is found in various parts of Africa, including East and West Africa. A new entrant species, *Anopheles stephensi* has recently invaded East Africa but is originally predominantly found in parts of Asia, including countries such as India, Pakistan, Iran, Iraq, Afghanistan, and Saudi Arabia [19]. These malaria mosquitoes exhibit some degree of geographic variation within their ranges and their distribution may not be limited to the countries mentioned above, as they may be present in neighboring regions.

Malaria transmission by *Anopheles* mosquito species in East Africa vary in density [20–22]. *An. gambiae* is often the most important vector in many parts of East Africa. It typically prefers breeding in sunlit water bodies such as small temporary pools, puddles, and agricultural fields. It is adapted to both rural and urban environments and exhibits a preference for biting humans. *Anopheles funestus* is another major malaria vector in East Africa, particularly in areas with higher rainfall and more permanent water bodies. It commonly breeds in large, permanent bodies of water like swamps, lakes, and rivers. It is typically more active during the early evening and late night hours, and it tends to bite both humans and animals. *Anopheles arabiensis* is known for its ability to adapt to various ecological settings. It exhibits a preference for breeding in temporary water bodies, including small rain pools, agricultural fields, and human-made containers. It is commonly found in rural areas and is often active during the early evening and early morning hours. *Anopheles merus* overall contribution to malaria transmission may vary depending on local conditions and its relative abundance compared to other species. *Anopheles coustani* plays a lesser role in malaria transmission compared to species like *An. gambiae* s.s. and *An. funestus*. It is more commonly found in savannah and grassland regions and exhibits a preference for biting both humans and animals [14].

#### **3.3 Areas of high transmission of** *Plasmodium* **parasites**

As depicted in **Figure 1** the areas of high transmission for *Anopheles* mosquito species vary within East Africa depending on factors such as climate, geography, and local ecological conditions [24]. The precise distribution and intensity of malaria transmission may vary within these regions, influenced by factors such as climate variations, human population movements, and the prevalence of malaria parasites [14]. Warmer temperatures shorten the development time of the *Plasmodium* parasites within the mosquito, leading to increased transmission rates. Changes in rainfall patterns experienced in parts of East Africa are responsible for creation of breeding sites for mosquitoes, affecting their abundance. The high population movements including travel in East Africa, has introduced malaria infected individuals to new areas triggering outbreaks of malaria especially in highlands where many susceptible individuals live.

The Entomological Inoculation Rate (EIR) referring to the number of infectious mosquito bites received by an individual per year is a crucial epidemiological measure used to assess the intensity of transmission of malaria. The EIR is higher in areas with high mosquito density, favorable breeding conditions, and high malaria prevalence in humans compared to areas with lower malaria prevalence [24, 25]. *Anopheles gambiae* is known to be highly efficient in transmitting malaria parasites, particularly *Plasmodium falciparum*, which remains the most common in the region and highly virulent. The EIR has been used in East Africa to gauge the risk of malaria infection faced by local populations. Surveys involving human landing catches and indoor mosquito trapping have been carried out to estimate mosquito biting rates and the proportions of mosquitoes infected with *Plasmodium* parasites [22, 26].

#### **3.4 Prevalence and diversity of** *Plasmodium* **species**

Malaria, caused by *Plasmodium* parasites, is a major public health concern in East Africa. The region is highly endemic for malaria, with transmission occurring throughout the year, although intensity may vary across different areas and seasons. The main species of *Plasmodium* responsible for malaria in East Africa is *Plasmodium falciparum*, which is the most virulent species and responsible for the majority of malaria-related deaths, and *Plasmodium vivax*, which is less prevalent but can cause relapses of the disease [24]. High parasite prevalence is bound to increase the chances of the *Anopheles* mosquitoes acquiring infection from human hosts during blood feeding and eventually transmitting it to other persons [27]. The prevalence of *Plasmodium* parasites in East Africa is influenced by several factors, including climate, geography, human behavior, and socioeconomic conditions. Malaria transmission is facilitated by the presence of competent mosquito vectors, primarily *Anopheles* mosquitoes, which breed in various types of water bodies, such as stagnant pools, swamps, and rice fields. The high humidity and temperature in many parts of East Africa provide favorable conditions for mosquito breeding and parasite development [28].

#### **3.5 Seasonality, altitude and related factors**

Seasonality, altitude, and other related factors play significant roles in malaria occurrence and transmission in East Africa [24]. Malaria transmission in East Africa often exhibits seasonal patterns, with higher transmission rates during certain times of the year. The exact timing and duration of the peak transmission season varies

*Malaria Transmission Dynamics in East Africa DOI: http://dx.doi.org/10.5772/intechopen.113192*

#### **Figure 1.**

Plasmodium falciparum *parasite prevalence in East Africa [23].*

across different regions and countries. Generally, the transmission increases during or after the rainy season when there is an abundance of mosquito breeding sites. The rainy season provides optimal conditions for mosquito reproduction and survival, leading to increased malaria transmission. However, in some areas with irrigation schemes or areas with perennial transmission, malaria may occur throughout the

year [13, 29]. Altitude has a significant impact on malaria transmission in East Africa. Generally, malaria transmission decreases as altitude increases. This is because the *Anopheles* mosquitoes, which are the primary vectors for malaria, have specific altitude preferences. These mosquitoes thrive in low-lying areas and are less common at higher altitudes due to the cooler temperatures and less favorable environmental conditions. Therefore, highland areas in East Africa, particularly those above 2000 meters, tend to have lower malaria transmission rates compared to lowland areas.

Several other factors influence malaria occurrence and transmission in East Africa, including vector species, climate change and socioeconomic factors. The presence and abundance of specific mosquito vector species may affect the transmission dynamics. Different *Anopheles* mosquito species have varying preferences for breeding sites, feeding behavior, and susceptibility to *Plasmodium* parasites. Human activities, such as agricultural practices, deforestation, and settlement patterns, have an impact on malaria transmission. Changes in land use and water management practices may create or modify mosquito breeding sites, increasing the risk of malaria transmission. Climate variability and change may influence malaria transmission patterns in East Africa. Changes in temperature, rainfall patterns, and humidity levels may impact mosquito breeding, parasite development, and the geographic distribution of both the vectors and the parasites [30, 31]. Poverty, limited access to healthcare services, and inadequate infrastructure may contribute to the persistence and burden of malaria. These factors may affect the availability and utilization of preventive measures, diagnosis, and treatment options, leading to higher malaria transmission rates [25]. The specific influence of these factors may vary within different countries and regions of East Africa.

#### **4. Malaria transmission trends**

**Figure 2** depicts general trends of malaria transmission in East Africa. The transmission in the region is a complex interplay of various factors, including an enabling environment and habitats for breeding consisting of suitable water bodies, landscapes, vegetation, altitude and the phenomena of climate change. The conducive breeding environment yields three efficient vectors of malaria: *Anopheles gambiae*, *An. funestus,* and *An. arabiensis* among others. Due to their efficiency, female *Anopheles* mosquitoes infected with *Plasmodium* parasites from a previous human blood meal develop a tight relationship in which the parasites undergo sexual reproduction, fertilization and develop from zygote into ookenetes in the midgut and migrate to the body cavity as oocysts which multiply and rupture to release sporozoites. The sporozoites migrate to the salivary glands. At the next available blood meal, the mosquito vectors pass on the infective sporozoites into a new human hosts' blood and into the liver in form of merozoites. Merozoite introduction into the bloodstream causes malaria. Both human and domestic animal hosts interact with the environment in a delicate balance where if not treated, newly emerged adult females acquire parasites afresh from infected individuals and the transmission continues unabated.

In order to determine the level of infection in relation to transmission of malaria in East Africa, various methods and data sources may be employed to provide data on prevalence rates, transmission intensity, and intervention efforts in the region. Existing data sources and detailed reports on malaria transmission are available with organizations such as World Health Organization (WHO), Centers for Disease Control Malaria Information and the Malaria Atlas Project (MAP) [32–34]. The Lake Basins and the coastal regions in East Africa are endemic for malaria. These regions

*Malaria Transmission Dynamics in East Africa DOI: http://dx.doi.org/10.5772/intechopen.113192*

**Figure 2.** *Malaria transmission in East Africa (by author).*

experience higher transmission rates of malaria attributed to the climate and vector abundance. To provide insights into reported malaria cases from local health facilities and determine trends and prevalence rates of malaria, analysis of epidemiological data from surveillance systems is procedural contributing to determination of the level of infection. Alongside, vector surveillance is often conducted to assess vector abundance and *Anopheles* distribution in the region. Monitoring *Anopheles* presence and behavior is crucial for understanding transmission patterns [35–37].

Blood sampling and laboratory analysis is carried out in local laboratories to measure *Plasmodium* parasite prevalence and to determine the prevalence of malaria parasites among human populations in the region through conducting surveys in affected areas [38]. Interventions such as Long Lasting Insecticide Nets (LLINs), Indoor Residual Sprays (IRS) and administration of malaria drugs are assessed to help determine the existing level of transmission and gauge their impact [39]. Collaboration between researchers and local health authorities is crucial in providing and updating data on insights on the overall levels of infection [27].

#### **5. Role of interventions and challenges**

Interventions have mainly aimed at controlling *Anopheles* mosquitoes using (LLINs) and IRSs, preventing the spread of *Plasmodium* parasites, and reducing incidence of the disease through improved access to diagnostics and treatment. Longlasting Insecticide Nets have proved effective at protecting individuals during sleep while IRS has targeted high transmission areas where *Anopheles* mosquito densities are high. These interventions are proving successful in some areas of East Africa, but

there are challenges and limitations experienced regionally that lead to their limitation in impacting reduction in malaria transmission.

#### **5.1 Transmission persistence**

Emergence of insecticide-resistant *Anopheles* mosquito populations is widespread in the region attributed to pyrethroids used in LLINs and IRS programs. This has led to interventions becoming less effective in controlling the mosquito populations and gradually malaria transmission has persisted in areas where insecticide resistance is prevalent [26, 40, 41]. The malaria parasites have also developed resistance to treatment by some anti-malarial drugs such as chloroquine, which hampers the effectiveness of treatment interventions and further translates into persistence of malaria in the local population. High transmission of malaria is evident in some remote and impoverished areas of East Africa where access to healthcare facilities for treatment is lacking and people are often not diagonized and prescribed appropriate antimalarial drugs. Delayed or inadequate treatment has contributed to resistant forms of malaria and persistence of transmission in the region.

The East African Community has embraced population movement for economic enhancement; however, this movement has contributed to the spread of malaria into new horizons or into areas where the disease had been controlled. Such movement and other behavioral tendencies including outdoor activities during peak mosquito biting hours of evening and early in the morning expose people and increase the risk of mosquito bites and subsequent malaria infection. In some cases misconceptions about malaria prevention and treatment contributes to delays in seeking appropriate healthcare, aggravating transmission [42]. Lack of community acceptance of an intervention due to local beliefs has hampered success of some interventions. For instance, inconsistent use of bed nets and failure to adhere to treatment regimens reduces the impact of these interventions and provides for further transmission. Factors such as socio-economic status, education, cultural superstitions and practices influence the uptake of these interventions [43].

#### **5.2 Changes in climate**

Changes in climate have influenced mosquito breeding patterns and spread of malaria. Climate change has resulted in the expansion of geographical ranges of *Anopheles* mosquito vector populations and *Plasmodium* parasites, contributing to persistent transmission. Climate Variability has affected mosquito breeding habitats and influenced the distribution and abundance of *Anopheles* mosquitoes. Changes in temperature and rainfall patterns create new opportunities for mosquito breeding and alter the timing of peak transmission seasons [44]. Temperature affects both the *Anopheles* mosquito and *Plasmodium*'s life cycle in the vector. The daily temperature variation influences the survival, development, and reproduction rates of mosquitoes. Stagnant water after increased rainfall provides ideal conditions for mosquito larvae to thrive. Humid conditions and elevated temperatures in the region result into faster development rates for the mosquito from egg to adult, which translates into more rapid population growth. Due to the foregoing climatic changes the extrinsic incubation period, which is the time required for *Plasmodium* parasites to complete development within the malaria mosquito before transmission to a new host, is shortened thereby perpetuating transmission.

#### **5.3 Vector behavior and outdoor transmission**

In areas of high bed net coverage, *Anopheles* mosquitoes in East Africa have exhibited changes in their feeding behavior, with an increasing proportion of bites occurring outdoors and at times when people are not protected by bed nets [45]. This behavioral resistance reduces the effectiveness of indoor-based interventions leading to residual malaria transmission. Research and surveillance efforts have been hampered in the absence of understanding the behavior of outdoor biting mosquitoes. Outdoor resting sites for *Anopheles* mosquitoes have created alternative breeding and resting sites, a pattern that continues to contribute to transmission in the absence of wearing protective clothing and using repellents, especially during times when bed nets may offer limited protection.

#### **5.4 Sub-optimal implementation of integrated strategies**

Lack of integration of interventions hinders their effectiveness when they are implemented in isolation [46]. A case in point in the region is when vector control measures in some areas are not accompanied with case management propagating transmission rates due to untreated cases alongside concurrent mosquito breeding. Limited impact on transmission is also evident when mosquito behavior, human behavior and changes in environmental conditions are not addressed in combination leading to suboptimal results. The risk of resurgence in cases of malaria is imminent in the region without a comprehensive approach in implementing interventions. Focus on one intervention is the cause of emergence of resistant *Anopheles* mosquitoes and *Plasmodium* parasites in some areas of East Africa which continues to reduce the long-term effectiveness of interventions [47].

#### **6. Conclusion**

Malaria, a life-threatening mosquito-borne disease caused by *Plasmodium* parasites, continues to be a major public health concern in East Africa. The region, comprising of the countries Kenya, mainland Tanzania, and Uganda, is characterized by ecological features, a typical tropical climate and landscape endowed with the Great Rift Valley, lakes and lake basins which contribute to creation of breeding habitats for *Anopheles* mosquitoes. Most of these areas are relatively close to human dwellings. The proximity of *Anopheles* mosquito breeding habitats to human populations varies depending on the specific geographic location and local conditions. In general, *Anopheles* mosquitoes tend to breed in areas near human settlements both in urban and rural settings and take advantage of readily available blood meals. The main vectors of malaria, which are *Anopheles gambiae* and *An. funestus* and *An. arabiesis*, transmit the causative parasites particularly *Plasmodium falciparum* and to a lesser extent, *P. vivax.* Specific mosquito species' presence and abundance varies within countries depending on local environmental factors. The transmission dynamics of *Anopheles* mosquito species are influenced by factors such as vector behavior, ecology, vector control interventions, host availability, and the prevalence of malaria parasites within the human population.

The contribution of domestic animals to *Anopheles* mosquito breeding and malaria transmission depends on local conditions, such as vector species, animal husbandry

practices, and malaria prevalence in both humans and animals. Factors such as population movements, inadequate healthcare infrastructure, poverty, and limited access to effective prevention and treatment measures contribute to the persistence of malaria in the region. Efforts to interrupt malaria transmission in East Africa target both the mosquito vectors and the human hosts. Human behavior significantly impacts malaria transmission in the region. Outdoor activities during peak mosquito biting hours and inadequate use of preventive measures, such as insecticide-treated bed nets, increase the risk of mosquito bites and subsequently lead to malaria infection.

The climate, especially rainfall patterns and temperature affects both the mosquito and the malaria parasite's life cycle. Rainfall patterns affect the availability of breeding sites for mosquitoes when stagnant water from rainfall provides ideal conditions for mosquito larvae to thrive. The daily temperature variation influence their survival, development, and reproduction rates with higher temperatures leading to faster development rates of mosquitoes from egg to adult, resulting in more rapid mosquito population growth. Daily temperature variations also influence human behavior pertaining to use of bed nets where people may be less likely to sleep under bed nets during hot nights, leading to increased exposure to mosquito bites.

The sub-region faces several challenges in its efforts to control malaria. These challenges include the emergence of drug-resistant malaria parasites, insecticideresistant mosquitoes, inadequate healthcare infrastructure, and limited resources for implementing comprehensive control programs. The lack of awareness about malaria prevention and treatment measures may also lead to delays in seeking appropriate healthcare, further exacerbating the disease's transmission. Overcoming these intervention challenges requires collaboration among researchers, policymakers, and local communities to develop context-specific and sustainable approaches for malaria control in East Africa. Additionally, continuous monitoring and adaptation of strategies are essential to address evolving challenges and ensure progress toward malaria elimination goals.

#### **Acknowledgements**

I acknowledge the support by my employer, Jaramogi Oginga Odinga University of Science and Technology (JOOUST), especially financially and granting me a portion of time within which I was able to contribute this chapter. I also wish to acknowledge the authors led by Victor A. Alegana for utilizing their map of *Plasmodium falciparum* prevalence in East Africa. I thank Wendy P.L.A. Davis for the thorough editorial work she rendered to this chapter.

*Malaria Transmission Dynamics in East Africa DOI: http://dx.doi.org/10.5772/intechopen.113192*

#### **Author details**

Fred Anangwe Amimo Jaramogi Oginga Odinga University of Science and Technology, Bondo, Kenya

\*Address all correspondence to: amimofa@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### **Chapter 5**

## Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites of the National Malaria Control Program (NMCP) in the Democratic Republic of Congo (DRC)

*Emery Metelo, Josue Zanga, Doudou Batumbo, Bien-aimé Mandja, Hyacinthe Lukoki, Arsène Bokulu, Trèsor Iluku, Narcisse Basosila, Emile Manzambi, Fiacre Agossa and Erick Mukomena*

### **Abstract**

In order to represent the different epidemiological facies that abound in the Democratic Republic of Congo (DRC), new sentinel sites were created. Before their operationalization, baseline evaluations of the bionomics and the insecticide resistance status of malaria vectors were conducted. Using Human Landing Catches (HLCs) and Pyrethrum Spray Catches (PSCs), sampled *Anopheles gambiae s.l.* were screened for the presence of *Plasmodium falciparum*. Larval surveys were organized to assess the sensitivity of wild *An. gambiae* to selected insecticides. Surveys on the community use of Insecticide-Treated Nets (ITNs), Surveys on the community use of Insecticide-Treated Nets (ITNs), were conducted. A total of 2238 *Anopheles* were collected. Including, 1802 (80.5%) by HLC and 436 (19.5%) by PSC. The majority of the samples were *An. gambiae* (98%) with very high average transmission entomological indices (density, Human Biting Rates (HBRs) and Entomological Inoculation Rates (EIRs)). These *An. gambiae* were resistant to selected insecticides at all sites. Households close to breeding sites were at high risk. Overall, ITN coverage was low (41.7%). Of these three sites, only Mweka presented a good coverage of 90%. Only Mweka presented a good coverage of 90%. The sentinel sites are located in the same epidemiological facies where the conditions for transmission of the disease and the incidence are identical. This transmission is ensured by *An. gambiae* with high resistance statuses vis-à-vis pyrethroids. The ecological choice is necessary for a good representation.

**Keywords:** *An. gambiae s.l.*, resistance, entomological indices, sentinel sites, DRC

#### **1. Introduction**

Malaria is a parasitosis that constitutes a global public health problem and is still endemic in many parts of the world with an estimated 247 million cases in 2021 [1]. It is particularly rife in Sub-Saharan Africa where 95% (234 million) of the global malaria cases and subsequently 96% (593,000) of all deaths occur. In this region of Africa, four countries (Nigeria 27%, the Democratic Republic of Congo (DRC) 12%, Uganda 5% and Mozambique 4%) pay a particularly heavy toll from malaria [1]. Nigeria and the DRC alone carry 39% of the global burden of malaria [1].

In the DRC, malaria remains a major public health problem, despite significant progress made [2, 3]. In 2021, the number of deaths associated with malaria was estimated at 22,729, of which 15,297 occurred of children under the age of 5, i.e., 67% [4, 5]. Around 97% of the population lives in areas with stable, endemic malaria transmission characterized by the equatorial and tropical facies [4]. This country has five distinct climatic zones (Equatorial zone, South tropical wet zone, South tropical dry zone, North tropical wet zone and Temperate tropical zone) [6]. The equatorial climate occupies the central basin and the tropical climate extends from north to south, thus occupying most of the country. The DRC stretches over an area of about 2,345,000 km2 , and it alone presents almost all the ecologies and bioclimatic strata encountered in Africa, from the Sahelian savannah regions to the equatorial forest regions [4]. This environmental heterogeneity makes combating malaria in the DRC particularly complex [6, 7]. Moreover, despite several rounds of mass distribution of ITNs, the country is struggling to achieve 60% ITN coverage [7].

This DRC has adhered to the World Health Organization's (WHO's) global technical strategy for the fight against malaria (2016–2030), which provides comprehensive technical guidance with the aim of reducing the incidence and mortality rates linked to malaria by at least 90% compared to 2015 reference year [1].

Large-scale use of ITNs offers good protection to populations at risk of malaria and has been the primary tool for vector control for nearly 40 years [8]. Taking this global strategy into account, for almost a decade the DRC has opted for building universal coverage of malaria control interventions within the National Strategic Plan (NSP) [4]. The current NSP adopted at the national level is aligned with the global technical strategy for the fight against malaria, with the objectives of reducing the rate of morbidity and mortality linked to malaria by at least 40% by the end of 2023 with coverage ≥80% in ITNs [4].

One of the guiding principles the NSP has set itself is the implementation of vector control based on evidence and convincing results [4]. To do this, the National Malaria Control Program (NMCP) relies on data from routine systematic surveys and studies [4, 7, 9] requiring the establishment of sentinel sites. Together, these sentinel sites generate inexpensive, complete and quality data [4]. They are distributed according to the health subdivision, Health Division (HD), Health Zone (HZ) and Health Area (HA), which is similar to the administrative subdivision (province, territory, community, village) seen elsewhere.

Parasitological, pharmacological and environmental data are needed to assess the evolution of malaria [4] and this guided how sentinel surveillance sites were created and located across the country on the basis of the Health Division (HD). Sentinel sites correspond to health surveillance zones where parasitological and entomological surveys are carried out to provide basic data for evaluating malaria control activities, and which represent the Province Health Division (PHD).

*Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

Before decentralization in 2010, the DRC had 11 provinces giving 11 sentinel sites. After decentralization, 26 provinces were obtained. Thus, the DRC will gradually add 15 new sites corresponding to one site in each of the 26 Provincial Health Divisions (PHDs) [10]. These sentinel sites represent the different epidemiological facies of malaria transmission found in the DRC.

It is important to note that the distribution of each *Anopheles* species relies on ecological characteristics favorable to their reproduction and multiplication [11, 12]. In the DRC, the most encountered vectors are *Anopheles gambiae*, *Anopheles funestus*, *Anopheles nili*, *Anopheles moucheti* and *Anopheles paludis* [4]. These species are not evenly distributed in all PHDs.

Surveys examining vector bionomics (behaviors including host preference, host-seeking, peak activity period as well as larval site characteristics) and insecticide resistance (IR) monitoring are carried out in the various sentinel sites for evidence that helps the NMCP choose the most appropriate vector control tools. The emergence of IR in the African region raises many questions about the ongoing effectiveness of ITNs in helping to reduce the incidence of malaria [8]. Increasing evidence of *Anopheles* resistance to different classes of insecticides is reported by several authors [9, 13, 14]. Thus, it is necessary that the choice of sentinel sites take into account the ecology and the bionomics of the vectors, as well as the profiles of the *Anopheles* vis-àvis the insecticides used.

The current choice of NMCP sentinel sites is based on administrative criteria alone and does not address the biodiversity of vector species and various epidemic facies. Indeed, the work of Rahm and Vermilin [15] on the distribution of anopheline species in all Congolese territories identified 66 species of *Anopheles*, whereas Watsenga et al.'s [9] work, based on sentinel sites in the DRC, identified fewer species. Yet, there is evidence elsewhere (Cameroon) that a greater diversity of species can be identified when the choice of sentinel sites is based on ecological criteria [16].

To enable the new sentinel sites to fully play their role by providing reliable data and reflecting the local reality of malaria transmission, this pilot study was carried out to assess entomological surveillance in the nine additional sentinel sites. Only four sites, namely Boende in the Province of Tshuapa, Lisala in the Province of Mongala, Mweka in Kasai and Nyakunde in Ituri, were selected.

The present study aims to contribute to our understanding of the local epidemiology of malaria by identifying vector species, determining entomological parameters (species, density, HBR and EIR) and the profile of *An. gambiae s.l.'s* susceptibility to the usual insecticides and the factors modulating the transmission of malaria in the sentinel sites.

#### **2. Complexity of vector control and entomological surveillance in endemic sentinel sites of the NMCP in DRC**

#### **2.1 Methods**

#### *2.1.1 Study sites*

The study was conducted in four new sentinel sites, from June 17 to July 17, 2019 for the first three sites (Boende, Lisala and Nyakunde) and from October 15 to November 15, 2019 for the Mweka site (**Figure 1**) by three teams of entomologists from Laboratory of Bio-ecology and Vector Control at the Kinshasa School of Public

#### **Figure 1.**

*Sentinel mosquito collection sites (in green, the provinces and in red the capture point).*


*3 S Too w, South Tropical Wet.*

#### **Table 1.**

*Entomological sentinel site location, period of data collection and climatic zone.*

*Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

Health (BIOLAV-KSPH). Location of entomological sentinel sites and frequency of sensitivity testing and trapping to determine human bite rates are given in **Table 1**.

#### *2.1.1.1 Boende*

Boende is located in the Province of Tshuapa, on the Tshuapa and Lomami rivers, east of Mbandaka, 1400 km north-east of the capital Kinshasa. It is served by Route Nationale 8.

#### *2.1.1.2 Lisala*

Lisala is located in Mongala province, on the right bank of the Congo River, in a forested area with an equatorial climate. It lies 2650 km north-east of Kinshasa and is served by the Route Nationale 6.

#### *2.1.1.3 Nyakunde*

Nyakunde is the seat of the Andisoma chiefdom of the Bira, located some 30 km south-west of Bunia, in the Irumu territory, Ituri district, Orientale Province in northeastern Democratic Republic of Congo. It comprises three regions with distinct climatic characteristics: a very rainy region like the equatorial basin, an intermediate zone where rainfall decreases during the dry season like the tropical zone, and a territory with little rainfall but alternating between the two seasons.

#### *2.1.1.4 Mweka*

Mweka is a decentralized entity in the Province of Kasai in the Democratic Republic of Congo. Located in a savannah zone with a humid tropical climate with two seasons (dry from May to September and rainy September to December), the territory is characterized by a slightly warm temperature with a maximum estimated at 27°C. The climatic elements (temperature and rainfall) for all three sites sampled at different stations (Lisala, Tshuapa and Tshikapa) are shown in **Figure 2**.

#### **Figure 2.**

*Ombrothermic curve of Mweka, Lisala and Boende in 2019 (source: Lisala, Tshuapa and Tshikapa weather station).*

#### *2.1.2 Study population and sampling technique*

The statistical unit of the study was the *Anopheles* mosquito specimens, which were collected from selected houses using a four-stage probability sampling technique:


However, one of the surveys was underway and it was possible to sample only in three sites (Boende, Lisala and Mweka) out of the four initially planned, i.e., a total of 60 houses instead of the 80. Unfortunately, security threats at the sentinel sites within Nyakunde prevented the surveys from going ahead as intended. Thus, this site was removed from our analyses.

#### *2.1.3 Data collection study procedure*

#### *2.1.3.1 Mosquito collection*

#### *2.1.3.1.1 Larva collection*

Larvae and pupae of *An. gambiae s.l.* were collected from different larval sites. After sorting, the selected larvae were reared in tanks covered with mosquito nets and then kept under conditions likely to favor the emergence of mosquitoes, without contamination of the rearing by external mosquitoes and without infestation of the environment. The daily monitoring of the farm consisted in counting the numbers of larvae in each tank, on the one hand, and living and dead imagoes in each cage, on the other hand [18].

#### *2.1.3.1.2 Adult mosquitoes (anopheles)*

Two collection methods were used: Capture by indoor spraying with pyrethrums (PSCs) allowing the residual density of resting anophelines and their trophic states to be established; and human landing catches (HLCs) to assess the risk of infective/ human/night bites, *Anopheles* behavior and the cycle of aggression (peak biting).

#### *2.1.3.1.3 Human landing catches (HLCs)*

Human landing catches (HLCs) were conducted in 10 houses per village from 6 p.m. to 6 a.m., and the data reported cover only 12 h of nocturnal captures. Thus, in each village, 10 houses were selected for capture, giving a total of 20 houses

#### *Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

per site and 60 houses for all sites. As mentioned above, the Nyakunde data were not taken into account. Four mosquito collectors were assigned to each house, i.e., two outdoor and two indoor. After sorting the mosquitoes morphologically to identify the anophelines in the field, the mosquitoes were individually preserved in Eppendorf tubes with silica gel and brought back to the BIOLAV/KSPH laboratory for further analysis (identification, determination of infectivity and WHO Tube Pass sensitivity test) [19].

#### *2.1.3.1.4 Capture by indoor pyrethrum spray (PSC)*

The PSC was carried out by spraying a pyrethrum-based insecticide in 10 non-HLC houses between 6 and 10 a.m. after removing cooking utensils, drinks and food. White sheets were spread out on the floor in all rooms of the house and spraying began outside the house, in front of doors and windows, then inside. A commercially available pyrethroid spray (Baygon, Bayer) was sprayed in the house and doors were closed for 15 min. After which, doors and windows were opened to collect mosquitoes that had fallen onto the sheets. Female mosquitoes were classified according to four trophic statuses (unfed, freshly fed, half-gravid and gravid) [17, 20].

#### *2.1.4 Household survey*

The household surveys were carried out in each of the houses where the mosquitoes were captured. Household heads were interviewed based on the questions administered in the Malaria Indicator Surveys (MIS). The questionnaire included questions on sociodemographic status, the notion of fever in children under 5 years of age, the use or not of mosquito nets and the physical condition of ITNs. The type of construction materials of the houses (blackberries and roofs) was also characterized in order to determine the quality of the habitat allowing the access or not of the mosquitoes [7].

#### *2.1.5 Data and studies procedure*

#### *2.1.5.1 Morphological identification of Culicidae*

The mosquitoes were sorted based on morphological criteria to gender and sex in the field. They were then stored individually in Eppendorf tubes with silica gel and sent to the entomology laboratory of the Kinshasa School of Public Health (KSPH). Once in the laboratory, morphological identification under the microscope was conducted using dichotomous keys [21, 22].

#### *2.1.5.2 Insecticide susceptibility tests and synergist bioassays*

Sensitivity tests were carried out on wild populations of *Anopheles* in each of the four sentinel sites selected. They made it possible to better know the levels of sensitivity of the vectors to the two Pyrethroids (Alphacypermethrin and Deltamethrin). And in the event of resistance to pyrethroids, a pre-exposure to Piperonyl Butoxide (PBO) was carried out for the search for oxidases. The effectiveness of these compounds was compared with that of DDT and Bendiocarb, to detect the existence or not of cross-resistance between the three chemical families. And this insecticide's

effectiveness was measured according to their knockdown effect (Kdt50 and Kdt95) and the mortality they cause after 24 h of observation.

Larval collections were made in each study site using larval dippers. Larval sampling was done in transient, sun-exposed puddles to maximize the likelihood of sampling *An. gambiae s.l.*. Larvae were subsequently transported to a field insectary for rearing. Emerging adult mosquitoes were provided with sugar solution until they were used for insecticide susceptibility tests [9].

Adult females 2–5 days old were selected and tested according to the WHO protocol [18]. The tests were carried out with the papers impregnated with insecticides from the WHO Kit composed of four types of insecticides and the PBO synergist of different concentrations: Deltamethrin 0.05%, Alphacypermethrin 0.05%, Bendiocarb 0.1%, DDT 4% and PBO 5%. And the effectiveness of insecticide was measured according to their knockdown effect for 60 min of contact and the mortality they cause after 24 h of observation.

The tests were carried out in accordance with the general conditions of the test procedures recommended by the WHO, particularly with regard to: temperature (between 23 and 27°C), relative humidity (between 70 and 90%), the age of the adult female mosquitoes (2 to 5 days), the number of times an insecticide-treated paper should be used (Not more than 6 times/At most 150 mosquitoes per paper for pyrethroids), the qualities and the number of uses of control test papers [18].

For each insecticide retained, four test replications, each involving 25 mosquitoes, were carried out. The first series of tests was carried out to observe the behavior of *An. gambiae s.l.* after 60 min of contact with insecticides. The numbers of mosquitoes shocked after 10, 15, 20, 30, 40, 50 and 60 min of exposure or contact with the insecticide were recorded, in order to calculate the shock effect (Kdt50 and Kdt95). Twentyfive mosquitoes were used for the negative control. After exposure, mosquitoes were transferred to clean holding tubes and provided with sugar solution. Mortality was recorded 24 h after exposure.

In the event of resistance to pyrethroids, a second series of tests was carried out with anophelines pre-exposed to PBO for 60 min.

The test results were interpreted according to the WHO criteria [18]:


#### *2.1.5.3 Determination of the infectivity rate by the enzyme-linked immunosorbent assay (ELISA)-plasmodium falciparum circumsporozoite protein (ELISA-CSP) pf technique*

After morphological identification, a subsample of individually stored *An. gambiae s.l.* collected by HLC were analyzed at the Institut National de la Recherche Biomédicale (INRB). The *An. gambiae s.l.* female were ground for analysis in the pf CSP ELISA test according to the Wurtz protocol.

The sporozoite index (SI) was determined after ELISA-CSP to detect the presence of the circumsporozoite protein of *P. falciparum*. However, after grinding Blocking Buffer (BB-IGEPAL®) using a pestle, the Pf monoclonal antibodies (mAb Pf) were fixed on the microplates, and the ground material was brought into contact with the

*Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

mAb Pf. In addition, the enzyme-coupled conjugate (mAb capture, peroxidase) was added before adding the enzyme substrate (hydrogen peroxide (H2O2)). Then, the reading was made with an ELISA reader at 405 nm.

#### *2.1.5.4 Determination of entomological indices of transmission*

#### *2.1.5.4.1 Indoor resting density (IRD)*

Density of resting mosquitoes inside houses was estimated by the total number of female mosquitoes resting indoors on the total number of houses inspected.

#### *2.1.5.4.2 Human biting rate (HBR)*

This rate designates the number of mosquito bites received by a person during one night (bite/human/night). It was estimated on the basis of the number of *Anopheles* caught on human bait, from 6 p.m. to 6 a.m.

#### *2.1.5.4.3 Entomological inoculation rate (EIR)*

The entomological inoculation rate (EIR) represents the average number of infective bites per human per night (ib/h/n). It is calculated as follows:

$$EIR = HBR \times \text{Sporozoite index} \left( \% \right) / 100. \tag{1}$$

#### *2.1.6 Data analysis*

Data were entered using Microsoft Excel® 2016. Statistical analyses of study data were performed using R® software in version 4.1.2. A statistical analysis was carried out on certain parameters with the usual statistical tests.

The data were essentially summarized in the form of a proportion because it was mainly necessary to calculate the entomological indices. Additionally, data were summarized in tables and graphs using Microsoft Excel® 2016 and R® 4.1.2. As the Shapiro test and the quantile-quantile (QQ ) graph showed that the distribution of our data was not normal, it was more the nonparametric tests that were used in the rest of the analyses. The results obtained in the six health areas during the different surveys were compared using Kruskal-Wallis tests on the one hand, for the comparison of the medians and Fisher exact to compare the proportions. To determine factors associated with anopheline density, a nonparametric Theil-Sen regression model was used. Before performing the regression, exploratory univariate analyses were performed to identify independent variables significantly associated with anopheline density. A two-sided alpha level of 0.05 was considered statistically significant.

#### **2.2 Results**

#### *2.2.1 Mosquito species composition and abundance*

A total of 6697 mosquitoes were collected by HLC and PSC. *Culex* were most abundant with 4459 (66.58%), compared to 2238 *Anopheles* (33.41%). Of the 2238 *Anopheles* collected, 1802 were collected by HLC and 436 by PSC.

Of the 436 *Anopheles* collected, *An. gambiae s.l.* was predominant with 430 specimens (98%). *An. funestus* and *Anopheles ziemani* were poorly represented with, respectively, five and one specimens (1.15 and 0.23%) (**Figure 3**). Mweka presented a higher abundance of 156 *Anopheles* specimens (35.78%) than the other two sites.

In view of the observations made by HLC, it emerges from **Figure 4** that 1802 mosquitoes were captured in the three sites. Indeed, *An. gambiae s.l.* showed a

#### **Figure 3.**

*Distribution of anopheles collected by PSC and HLC according to three sentinel sites.*

#### **Figure 4.**

*Biting time of an. Gambiae s.l. collected by HLC according to sentinel sites. (a) Lisala (As pêcheur), (b) Boende (As motema-mosantu), (c) Mweka (As Mweka), (d) Lisala (As antenne), (e) Boende (As Boende II), (f) Mweka (As Mweka). Out = outdoor; In = indoor.*

#### *Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

remarkable abundance of 1796 specimens (or 99.67%). The remaining two species (*An. funestus* and *An. ziemani*) were poorly represented with five and one specimens (i.e., 0.28 and 0.06%) (**Figure 3**). The Mweka site was the most abundant site in*Anopheles*, with 664 specimens (36.85%).

#### *2.2.2 Determination of entomological parameters*

#### *2.2.2.1 PSC collection*

With regard to the results presented in **Table 2**, the Resting Density Index (RDI) and the Indirect Human Biting Rate (IHBR) observed vary greatly from one site to another, and even from one village to another. Thus, the average values observed varied from 4.9 to 10 *An*./house for the RDI and from 0.08 to 1.55*An*. fed/human/night for the indirect bite.

The highest and lowest RDIs were, respectively, observed in the Buza district (Boende II/Kimbangu) and Hôpital (Motema-Mosantu) from the Boende site (Tshuapa) with 10 and 4.9*An*. female/house, respectively.

As for indirect Human Biting Rate, the Buza district (Boende II/Kimbangu) presented the highest indirect bite rate next to the 3C district (Antenne), which presented the lowest indirect bite rate with, respectively, 1.55 and 0.08*An*. fed/human/night.

#### *2.2.2.2 Human landing catch (HLC)*

#### *2.2.2.2.1 Biting time*

Overall, anopheline biting activity started very early at 6 p.m., initially increasing gradually until 10 p.m. where the numbers captured increased at a higher rate before peaking between 24 a.m. and 2 a.m. The exception was Bulongo village where biting activity peaked notably later at 2 a.m. (indoors) and numbers captured began to decrease around 5 a.m. (**Figure 4**). The behavior of *Anopheles* sp. was more endophagic at all sites.

#### *2.2.3 Entomological clues of transmission*

It appears from **Table 3** that the average of the entomological indices of transmission (density, HBR, SI and EIR) was high and varied from one site to another. High


*\*RDI: Relative density indoor (mean numbers of Anopheles per house). IHBI: Indirect Human Biting Rate (Anopheles fed/human/night).*

#### **Table 2.**

*Entomological evaluation indices by PSC in sentinel sites.*


*\*HBR: Human Biting Rate (bite/human/night); SI: sporozoite index; EIR: entomological inoculation rate (infective bites/human/night).*

#### **Table 3.**

*Synthesis of entomological indices of transmission.*

density was recorded at Tshuapa Boende in Boende II health area (10 *Anopheles*/ house), followed by Kasai Mweka in Mweka 1 health area (9 *Anopheles*/house), and low density was recorded in Tshuapa Boende in the Motema-Mosantu health area (4.9 years/house). The HBR was more recorded indoor (13.1 b/h/n) against 9.3 b/h/n outdoor. However, in Kasai Mweka in the health area of Mweka 1, the high rate of bites was recorded (19.8 b/h/n) and the same EIR (2.73 ib./h/n).

#### *2.2.4 Anopheles susceptibility profile of gambiae s.l. against common insecticides*

The results of WHO susceptibility tests carried out on wild populations of *An. gambiae s.l.* in the four sites (Tshuapa/Boende, Mongala/Lisala, Mweka/Kasaï and Ituri/ Nyakunde) to the two pyrethroids (Alphacypermethrin and Deltamethrin), to the organochlorines (DDT) and to the carbamates (Bendiocarb) are recorded in **Figure 5**.

#### **Figure 5.**

*Percentage mortality of an. Gambiae s.l. after pre-exposure to PBO followed by permethrin 1X and Alphacypermethrin 1X in WHO tube tests in four sites.*

*Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

Susceptibility tests in indicated generally reveal that, across all collection sites, *An. gambliae s.l.* showed resistance (≤90%) of varying degrees to Alphacypermethrin and Deltamethrin. Separately, Deltamethrin mortality was slightly elevated at all sites in contrast to that of Alphacypermethrin. After pre-exposure of *An. gambiae s.l.* to PBO, the efficacy of Deltamethrin and Alphacypermethrin was fully restored at all sites to 98–100%.

**Figure 5** shows that *An. gambiae s.l.* tested were more resistant to DDT with very low mortality of 11% in Mweka and 19% tied in Boende and Lisala. Only Bendiocarb was 100% effective at all sites.

**Table 4** presents the analysis that determined the Kdt50 and Kdt95, the value of these two parameters Kdt (knockdown time) corresponds to the time after which,


*\*R = resistance (24 h mortality ≤90%); S = sensibility (mortality 24 h 98–100%), RP = Resistance probability (mortality 24 h between 90 and 97%), n/a = not available.*

#### **Table 4.**

*Knockdown time of an. Gambiae s.l. after 60 min exposure to insecticides and mortality after 24 h.*

respectively, 50 and 95% of *An. gambiae s.l.* were knocked out after 60 min of insecticide contact. Kdt50 and Kdt95 have never been reached in all sites with DDT. The Kdt50 with Deltamethrin and Alphacypermethrin was not reached in the only site of Nyakunde and elsewhere, it was reached late.

Kdt95 with Deltamethrin and Alphacypermethrin was reached with difficulty and late at the Tshuapa/Boende site, respectively, at 44.7 min 95% confidence interval (CI) (41.9–48.3 min) and 49.6 min 95% CI (45.6–53.3 min). After pre-exposure to PBO, the efficacy of these two pyrethroids was restored and Kdt50 and Kdt95 were reached.

It is observed in Mweka, Kdt95 was not available to Deltamethrin and reached late forAlphacypermethrin by 50.3 (45.9–56.1) min, which reflected a high degree of resistance of *An. gambiae s.l.* to these two pyrethroids. However, in Nyakunde, Kdt95 was not available to Deltamethrin and Alphacypermethrin.

#### *2.2.5 Infectivity rate*

Sporozoite rates for all three sentinel sites are presented of 440 *An. gambiae s.l.* analyzed, the mean sporozoite rate in all sites being 11.6% (95% CI 3.4–5.2) for *An. gambiae s.l*. Moreover, sporozoite rates were relatively low at all sites. The lowest sporozoite rate was observed in Lisala at 9.8% (95% CI 1.1–4.6) and the highest at Mweka at 13.1 (95% CI 1.1–4.6).

#### *2.2.6 Determination of factors modulating malaria transmission in sentinel sites*

**Table 5** shows that in the Antenne health area, out of the 10 houses surveyed, no engorged *Anopheles* were found, whereas the median value was higher in Motema-Mosantu, Boende 2 and Pêcheur. The difference in the medians in the six health areas is statistically significant (p = 0.038). The distance between houses and larval sites was shorter in the health area of Motema-Mosantu compared to the other health areas (p < 0.001). The quality of mosquito nets shows a significant difference between HAs (p < 0.001), with 0 and 1% for good quality of mosquito nets in Antenne and Mweka 1 HAs. The survey showed a difference in the proportions of metallic houses (p < 0.001), with 0% in AS Pêcheur, Bulongo, Mweka 1 and Boende 2 (**Table 5**). Possession of ITNs was very low in Lisala and Boende at 15 and 20%, respectively, and in Mweka there was good coverage of 90% with an average possession in the sites of 41.7%.

**Table 6** shows that the anopheline density, according to our data, is significantly lower for houses located far from breeding larval sites (protective role because the odds ratio is less than 1). The other two variables were not statistically significant. The anopheline density with engorgement, according to our data, is significantly high for houses with a high number of ITNs.

The exponential of these coefficients gives the odds ratio, the distance between the household and the breeding site; the number of people in the household; and house type.

#### **2.3 Discussion**

#### *2.3.1 Identification of vector species*

After the staggered capture over the duration of the study, three species of *Anopheles* were identified overall but with a remarkably high density of *An. gambiae s.l.* found at all sites. However, according to Mouchet et al. [23], the different species of *Anopheles* exploit a wide variety of water collections as roosts, in particular the residual pools of




 **5.** *Factors influencing malaria transmission in sites.* *Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*


*\*Coefficients of the linear predictor of the resulting line of the logistic model. The exponential of these coefficients gives the odds ratio, the distance between the household and the breeding site; the number of people in the household; and house type.*

#### **Table 6.**

*Determination of factors influencing anapheline density.*

sunny stagnant surfaces, pools with erect vegetation, brackish water, etc. But *An. gambiae s.l.* preferentially uses residual pools of sunlit stagnant surfaces in several regions of Africa [24]. It is important to point out in the DRC with the advent of decentralization which increased the number of provinces from 11 to 26, a lot of modernization and urbanization work has been carried out in the new provinces to accommodate the provincial institutions.

Overall, correct urbanization reduces the risk of malaria transmission [1]. However, this urbanization has been anarchic in many provinces and has favored focal transmission of malaria, which results in a high disease burden [1, 4]. This anarchic urbanization is exacerbated by human activities which create temporary and artificial shelters favorable to the development of *An. gambaie sl* throughout the year [11, 12].

All urban areas of the DRC have almost the same ecology (large sunny space and the larval sites created by human activities). This ecology is favorable for the development of *An. gambiae s.l*. These results would reflect a low diversity observed. On the other hand, these results do not corroborate those of Rahm and Vermylen [15] and Sinka et al. [25], who found 61 species at the ecological scale of all the territories of the DRC.

#### *2.3.2 Determination of entomological parameters*

In general, the entomological indices of transmission are very high in the three capture sites. Despite high anopheline densities observed in Boende (Boende 2 health area) at 10 *An*./house and in Mweka (Mweka health area) at 9 *An*./house, compared to the Lisala site, no significant difference was observed. These highly observed densities can be justified by the poor quality of the habitat observed in these sites [26]. In addition, the absence of ITNs and the presence of larval sites also positively influenced anopheline density.

And we observed a high rate of *Anopheles* engorgement in the different households. The Boende site recorded more gorged *Anopheles* due to the absence of mosquito nets and a lot of shelter. There is a significant difference between distance from houses and larval sites. The houses of the Antenne site were very distant from the larval sites. Anopheline density was lower in houses located far from larval sites (protective effect).

The anopheline density of gorged *Anopheles* mosquitoes was twice as high in houses with more impregnated mosquito nets (poor quality of mosquito nets found).

The contraction on the effectiveness of ITNs is justified by the fact that ITNs were dilapidated and also the observed low durability [27]. In the Boende and Lisala sites, the ITNs' distribution campaign took place in 2016 (problem with the quality of ITNs found in households). The other two variables were not statistically significant.

It is observed that malaria transmission is intense in Mweka with a very high entomological HBR index of 19.8 bites/human/night and EIR index of 2.73 infectious bites/human/night. and This contrasts with the good coverage in ITNs of 90% recorded in 1 year after the mass distribution of ITNs. There is also a strong resistance of *An. gambiae s.l.* to Alphacypermethrin (61%) and Deltamethrin (56%). These observations corroborate those of Metelo et al. [28] in Bandundu-ville and Apinjoh et al. [29]. These observations require a study to evaluate the effectiveness of ITNs in the context of malaria endemicity and resistance of *An. gambiae s.l.* to insecticides.

*Anopheles* behaviors were more endophilic and endophagic in all collection sites. The aggressive cycle started early around 6 p.m. and peaked around 12 a.m. This activity remained intense all night with high rates of aggressiveness, infectivity and entomological inoculation rate in all sites. It emerges that these sentinel sites, which are located at different locations and climatic conditions, thus present the same condition of transmission of malaria and the same incidence.

#### *2.3.3 Anopheles susceptibility profile of gambiae s.l. against common insecticides*

For more than a decade, the NMCP has been carrying out mass distribution campaigns for ITNs in order to obtain wide coverage (≥80%) for effective protection of the vulnerable population [4]. The widespread use of ITNs must require regular monitoring of the sensitivity of wild *Anopheles* populations to the insecticides used in the fight for resistance management [30]. This is the subject of the creation of sentinel sites for monitoring malaria vector bionomics and resistance [4].

It emerges from our study that DDT was ineffective in all sites and no knockdown effect (Kdt50 and Kdt95) was achieved. The populations of *An. gambiae s.l.* in all sites exhibited high resistance (11–19%). These results corroborate those of Basilua et al. [13], Watsenga et al. [9] and Metelo et al. [31]. This is justified by the fact that these sites were agricultural sites at the time and benefited from the use of pesticides.

Deltamethrin and Alphacypermethrin reached Kdt50 late almost everywhere, except in Nyakunde. Deltamethrin was less effective compared to Alphacypermethrin. Anyway, the two pyrethroids (Deltamethrin and Alphacypermethrin) were less effective and the wild populations of *An. gambiae s.l.* have been resistant to them. Curiously after pre-exposure of *An. gambiae s.l.* to PBO, the efficacy of these two pyrethroids was restored at all sites.

#### **3. Conclusions**

The transmission of malaria is intense in the sentinel sites of the NMCP and is located in the same epidemiological facies where the conditions of transmission of the disease and the incidence are identical. This situation is aggravated by noncompliance with ITN distribution cycles and poor durability. This transmission is ensured by the *An. gambiae s.l.* with high resistance statuses to pyrethroids (Deltamethrin and Alphacypermethrin). The ecological choice is necessary for a good representation.

*Complexity of Vector Control and Entomological Surveillance in Endemic Sentinel Sites… DOI: http://dx.doi.org/10.5772/intechopen.114044*

In addition, this study will allow the development of mapping and will serve as a benchmark for future entomological assessments to improve malaria surveillance in the DRC.

#### **Acknowledgements**

Our sincere thanks to Prof. Dr. Jean-Jacques Muyembe, Director General of the National Institute of Biomedical Research, for his support. Thank you teams of entomologists from INRB and KPSK for their support. We are very grateful to Marianne Sinka for correcting the English grammar.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Emery Metelo1,2,3\*, Josue Zanga4 , Doudou Batumbo2 , Bien-aimé Mandja2 , Hyacinthe Lukoki1,3, Arsène Bokulu1,3, Trèsor Iluku1,4, Narcisse Basosila3,5, Emile Manzambi1 , Fiacre Agossa6 and Erick Mukomena5,7

1 Institut National de Recherche Biomédicale, Kinshasa, RD Congo

2 Faculté de Médecine, Université de Bandundu, Bandundu-Ville, RD Congo

3 Faculté de Sciences et Technologie, Université de Kinshasa, Kinshasa, RD Congo

4 Faculté de Médecine, Université de Kinshasa, Kinshasa, RD Congo

5 Hygiène et Prévention Programme National de Lutte contre le Paludisme, Ministère de la Santé Publique, RD, Congo

6 PMI VectorLink Project, Abt Associates, Rockville, MD, USA

7 Faculté de Médecine, Université de Lubumbashi, Lubumbashi, RD Congo

\*Address all correspondence to: pathymetelo@gmail.com

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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